It is known in the art of coil coating that structured surfaces can be obtained by changing the chemical composition of the paint (see e.g. the papers by P. Kunze in Oberflaeche-JOT, 1991(7), pages 40, 42 and 43, and by L. Jandel presented at the Workshop Coil Coating of the BSHG AK Oberflachentechnik held on 30.06.94 at Giengen, Germany). Paint as used herein defines as well primers as topcoats or even tie coats.
The incorporation of large particles (i.e. of a size larger than the thickness of the dry paint layer) in the paint, in the form of inert or reactive pigments or fillers, gives a structured coating depending on the particle size.
The incorporation in the paint of an additive which provokes an incompatibility (whether in the paint or during the curing process) also gives a structured surface depending on the incompatibility.
EP-A-47508 to SCHRAMM Lacke discloses the use of a finely divided polyamide suspended in a thermosetting paint with a hydroxyl-functional binder and a blocked polyiscyanate.
EP-A-288294 to BASF Lacke+Farben discloses the use of 0.01 to 0.05 wt % of polyethylene wax having a softening point of 100-120.degree. C. and a particle size of 5-35 um in the primer of certain polyester systems to obtain a structured finish.
DE-A-4019318 to BASF Lacke+Farben discloses the use of certain siloxanes in one or two-coat polyester compositions to obtain a structured surface.
The state of the art technology has many drawbacks. The main drawback is that changing the pattern of the structure requires changing the composition of the paint. Also, raw materials variability makes it difficult to reproduce the same structure from batch to batch. In addition, the structure pattern can in many cases be different in the web direction and in the transverse direction. Further, the systems based on the creation of an incompatibility are inherently subject to storage stability problems.
Mechanical means for making structured surfaces have already been proposed.
U.S. Pat. No. 5,565,260 discloses a method for applying polymer resin having a solids content of at least about 50 wt vol % with a coating roll having grooves in its surface.
U.S. Pat. No. 3,207,617 discloses a method for painting an embossed pattern on individual sheets of plywood, hardboard and the like, comprising applying a coat of liquid paint, mixing sand with said paint, and rolling a grooved roller to provide an embossed pattern.
FR-A-2,289,353 discloses a process for making an embossed coating on a substrate, wherein means for embossing are applied on a coating of a composition having a viscosity of 20 to 1,000 poises (dPa.s) whilst said coating in drying.
GB-1,512,967 discloses a process for making a decorative relief finish on a substrate such as a wall, using a pressing roll made of polyvinyl alcohol or polyvinyl acetal. The materials used to form the coatings are conventionally used in the building art to form thick films; exemplified materials have viscosities of 150 and 480 poise (dPa.s).
EP-A-79,759 discloses a method for providing surface replication in a coating on a sheet of material, wherein the coated sheet is at least partially set while it is pressed against a replicative surface. Such a process could not be adapted to coil coating because the continuous sheet must pass horizontally in the ovens
These mechanical means have one or more of the following drawbacks:
they demand the use of highly viscous coating compositions, which could not be used in coil coating; PA1 they cannot be used continuously, or where they can they have a low speed that cannot compare with the high speed of coil coating; and/or PA1 the structure has to be imparted during the drying step, what is obviously impossible in a coil coating oven. PA1 (a) applying a wet layer of a paint on a substrate, the paint having a viscosity of 30 to 200 s (as measured with a DIN 4 cup using DIN 53211 standard-method) and the substrate moving at a speed of at least 60 m/min; PA1 (b) passing the wet layer of paint under a roll having a raised pattern on its surface that imparts a structure to the wet layer; then PA1 (c) heating the thus coated and rolled substrate to form the structured surface.
There is thus a need in the art for a process for generating structured surfaces in coil coating which would not suffer from those drawbacks.
It is thus an object of the invention to provide a process for generating structured surfaces in coil coating that allows to change the structure pattern without having to change the paint composition.
It is another object of the invention to provide a process for generating structured surfaces in coil coating that will be essentially independent from the usual raw materials variability.
A further object of the invention is to provide a process for generating structured surfaces in coil coating that will not create paint storage stability problems.
These and other objects can be achieved by the process of the invention.
The process of the invention for generating a structured surface in coil coating comprises the successive steps of:
Coil coating is a continuous coating process of metal coils. The metal is generally steel, galvanised steel or aluminium. The coating is applied continuously on the metal moving at high speed, using roller or spraying applications and dried or cured by passing in a heated oven. The process involves metal pretreatment (such as degreasing, brushing, rinsing and chemical treatment, as may be required or desired) followed by the application and heating of two layers of coating (generally a primer and a topcoat). The coated metal can then be cut and given its final shape. Description of the coil coating process can be found in many textbooks, e.g. in "Organic Coatings: Science and Technology", vol.II, pages 290-5, Wicks et al., eds., Wiley, 1994.
The process of the invention can be applied to the primer and/or to the topcoat (or to any intermediate coat). When a primer is structured, it is known that it can be overcoated with a topcoat which essentially retains the structure imparted to the primer coat.
The wet layer of paint can be applied by any means used or useable in coil coating, such as bar coating, roller coating, curtain coating or extrusion. The wet film usually has a thickness of from 5 to 60 .mu.m; for primers the wet film thickness is preferably of about 20 .mu.m, while for topcoats it is preferably of about 50 .mu.m.
The invention is applicable to any type of paint that can be used in coil coating, be it of the heat curing or of the heat drying type. As examples of binders, there can be cited epoxy resins, polyesters, polyurethanes, polyesterurethanes, silicone-polyesters, PVC plastisols and organosols, polyvinylidene fluoride (PVdF) homo- or copolymers, and hybrid formulations as well as mixtures. Paint formulation in coil coating is known in the art and need not be described herein.
As in any other coil coating process, the paint properties (such as the thixotropy, the viscosity, the curing reaction) need to be adapted to the specific operating conditions. The viscosity of the paint is typically in the range of 30 to 200 s (as measured with a DIN 4 cup; DIN 53211 standard method), preferably 50 to 150 s, more preferably 60 to 120 s.
The heating step of the coated substrate to cure the paint is well known in the art; more particularly, one of ordinary skill in the art knows how to adapt the heating conditions.
The wet layer of paint is passed under a roll having a raised pattern on its surface in order to impart a structure to the wet layer before heating it to cure the paint. The roll may be made of any material, such as steel, stainless steel, brass, rubber, or it may even comprise a rubber coating on a metal roll. The diameter of the roll is easily adapted to the linear speed of the coil; the diameter is typically of from 10 to 2000 mm.
The raised pattern may be regular or irregular, any pattern design may be used. Typical dimensions are in the range of from 0.2 to 100 mm for the pattern unit and independently from 0.1 to 50 mm for each of the raised and hollow parts of the pattern. One of the advantages of the invention over the prior art is that it allows to obtain patterns of practically any unit size, whereas the prior art only provides small unit sizes (lower than 1 mm).
The depth of the pattern can be of from 0.01 to 10 mm, depending on the desired structure.
The distance between the surface of the raised pattern and the adjacent surface of the substrate is adapted to the desired depth of the structure, with the usual accuracy required in coil coating.
The structure created in the wet layer of paint is fixed by the curing process in the heating step. Said heating step must follow within a short period of time, which essentially depends on the viscosity and thixotropy of the wet paint; the speed of the coil in the process of the invention being of at least 60 m/min, preferably at least 80 m/min, there is no difficulty in placing the roll sufficiently close to the location where the paint will be cured, so that the paint viscosities and thixotropies usual in coil coating can be used.
The invention is thus suitable for all standard primer, intermediate and top coating compositions that can be used in coil coating.
The process of the invention provides a coil coating system which is simple to use and does not require any modification of the composition of the paints. In addition, the pattern of the structure can be varied practically at will and without interfering with the properties of the coating, what is impossible when varying the composition of the paints
A coil coating apparatus essentially comprises the following elements, in that order: a decoiler, a stitching or welding unit, an entry accumulator, a pretreatment zone, a first coating unit followed by an oven, a second coating unit followed by an oven, an exit accumulator, and a recoiling or shearing and stacking equipment.
The present invention also provides a coil coating apparatus characterised in that it comprises, between a coating unit and the oven following it, a roll having a raised pattern on its surface that is adapted to impart a structure to the wet layer of coating before it is heated.
This invention provides the first opportunity to adjust in a coil coating process the general image of the structure pattern, the dimension of the unit of a repeated motive and the slope of each single motive.